Thermal degradation of electrical properties and morphology of bulk single-crystal ZnO surfaces

Bulk ZnO substrates were rapid thermal annealed in either air or N ₂ at temperatures up to 500°C. The root-mean-square roughness of the surface as measured by atomic force microscopy begins to increase even after 200°C anneals in N ₂ or 300°C in air. The Schottky barrier height, Φ _B, obtained from Pt/Au diodes fabricated on these surfaces shows a decrease in effective barrier height upon annealing from 0.35 eV in control samples to ∼0.30 eV in those annealed at 400°C and an increase in the saturation current density (from 4 A cm ^-2 in the control samples to 17-37 A cm ^-2 in the annealed samples). These results show that the ZnO surface is degraded by quite low annealing temperatures and care must be exercised in designing the thermal budget for processing of ZnO devices. At much higher annealing temperatures (1000°C), the surface completely decomposes to leave metallic Zn droplets.